1. Field of the Invention
This invention relates to a high temperature inorganic binder. More particularly, this invention relates to an inorganic chemical binder which may be used in conjunction with refractory fibers at very high temperatures. In still greater particularity, the invention is an inorganic binder including a tackifying agent. By way of further characterization, but not by way of limitation thereto, the invention is a chromium aluminum phosphate binder including bentonite clay.
2. Description of the Prior Art
Prior art binders for bonding refractory fibers have included a variety of compounds including colloidal silica and phosphoric acid (actually P.sub.2 O.sub.5 which may be derived from H.sub.3 PO.sub.4 under appropriate temperature conditions). While these binder systems are useful in certain applications, they are not without relative disadvantages. For example, prior art phosphoric acid binders do not adhere well to refractory fibers. Binders including colloidal silica are subject to irreversible precipitation of the silica if the binder has been subjected to freezing temperatures.
A very serious disadvantage with prior art binder systems is binder migration. That is, as the binders are cured, the binder migrates to the surface of the material producing a material with a very soft interior. This causes serious integrity problems with the resulting material. The material is thus not suitable for use in applications requiring a monolithic structure. In addition, prior binder systems employing colloidal suspensions such as colloidal silica or alumina must be cured very carefully. That is, because of binder migration, the colloidal sol must be allowed to gel prior to curing. Failure to do so results in migration of the binder particles from the interior to the surface of the insulation layer. In addition, prior art binders must be sprayed onto a cool surface thus necessitating relatively long cooling periods before a furnace can be insulated. Another disadvantage of prior art binders has been that they are not nearly as temperature resistant as the refractory fibers themselves. Thus, high temperature applications of the fiber-binder mix are limited by the properties of the binder rather than the fiber. For example, an insulation product made from refractory fibers which are themselves temperature resistant up to 2600.degree. F. may only be applied in temperature environments up to 2000.degree. F. if the binder used in that product is only effective up to 2000.degree. F. Any reaction of the binder and fiber which tends to flux the fiber composition causes increased fiber shrinkage. Thus, the use temperature of the fiber--binder system is limited to a temperature less than the use temperature of the fibers alone.
Other practical problems encountered with the use of prior binder systems is that they are expensive, there may be safety and environmental problems in their applications, they may lack good suspension properties, and finally, as with prior phosphoric acid binders, the binders do not possess the tackiness or adhesiveness properties which are desirable in many applications. In applications where it is desirable to spray the refractory fiber onto a surface such as a furnace wall, it is essential that the binder be sufficiently tacky to cause the fibers to adhere to one another and to the surface.